builtin: improve docs for make slice

Fixes #22764

Change-Id: I339d3bbb08983e0d69d69a1d00c4d15d0661762a
Reviewed-on: https://go-review.googlesource.com/78335Reviewed-by: Rob Pike <r@golang.org>

src/builtin/builtin.go

@@ -171,8 +171,9 @@ func cap(v Type) int
 //	Slice: The size specifies the length. The capacity of the slice is
 //	equal to its length. A second integer argument may be provided to
 //	specify a different capacity; it must be no smaller than the
-//	length, so make([]int, 0, 10) allocates a slice of length 0 and
-//	capacity 10.
+//	length. For example, make([]int, 0, 10) allocates an underlying array
+//	of size 10 and returns a slice of length 0 and capacity 10 that is
+//	backed by this underlying array.
 //	Map: An empty map is allocated with enough space to hold the
 //	specified number of elements. The size may be omitted, in which case
 //	a small starting size is allocated.